Processes For Preparing 2,5-Furandicarboxylic Acid and Esters Thereof

ABSTRACT

A process for producing furan dicarboxylic acid or an ester thereof from a feedstock comprising hydroxymethyl furfural (HMF) and humins is disclosed. Humins are a byproduct from reactions forming HMF from sugars and are typically removed from the HMF prior to any further processing. A humins-containing HMF feedstock is utilized to produce furan dicarboxylic acids and ester substantially free from humins.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Non Provisional applicationSer. No. 17/068,206, filed Oct. 12, 2020, which is a continuation ofU.S. Non Provisional application Ser. No. 16/447,153, filed on Jun. 20,2019, which is a continuation of U.S. Non Provisional application Ser.No. 16/229,341, filed on Dec. 21, 2018, which is a continuation of U.S.Non Provisional application Ser. No. 15/743,015, filed on Jan. 9, 2018,which is a 371 of PCT/US2016/043296, filed Jul. 21, 2016 which claimsbenefit of priority from U.S. Provisional Application No. 62/196,808,filed on Jul. 24, 2015, all of which are incorporated herein byreference in their entirety.

FIELD OF THE DISCLOSURE

The present disclosure is directed towards processes for preparing2,5-furandicarboxylic acid and esters thereof, especially diestersthereof, from mixtures comprising 5-hydroxymethyl furfural and humins.

BACKGROUND OF THE DISCLOSURE

Poly (trimethylene furandicarboxylate (PTF) is a renewable polyester andcan be synthesized via the polycondensation reaction of2,5-furandicarboxylic acid or 2,5-furandicarboxylic acid diester with1,3-propane diol. Compared with polyethylene terephthalate (PET), PTFdemonstrates improved oxygen and carbon dioxide barrier properties thatare very important for the carbonated beverage and food packagingindustry.

The quality of the 2,5-furandicarboxylic acid diester, especially thecolor of the monomer is important for obtaining the high quality,colorless PTF that is required for the beverage, food and packagingindustries. The process for producing 2,5-furandicarboxylic acid and itsdiesters in a renewable manner proceeds via the oxidation ofhydroxymethyl furfural (HMF). HMF from renewable resources is oftencontaminated with highly colored polymeric impurities called humins. Theremoval of humins from processes producing furandicarboxylic acid andits diesters continues to be a problem.

The present disclosure relates to efficient processes for producing bothfuran dicarboxylic acid and its diesters that are substantially freefrom humins, from hydroxymethyl furfural contaminated with humins.

SUMMARY OF THE DISCLOSURE

The present disclosure relates to a process comprising:

-   -   a) oxidizing a feedstock comprising HMF and humins to produce a        mixture comprising crude humins-containing FDCA;    -   b) separating the mixture to obtain a solid FDCA/humins        composition;    -   c) esterifying the solid FDCA/humins composition with an alcohol        to produce a crude ester of FDCA; and    -   d) purifying the crude ester of FDCA obtained in step c) by        distillation or by sublimation to produce a purified ester of        FDCA substantially free of humins

In other embodiments, the process comprises:

-   -   i) oxidizing a feedstock comprising HMF and humins in a solvent        to produce a mixture comprising crude humins-containing FDCA;        and    -   ii) filtering the mixture under high temperature to obtain a        filtrate comprising FDCA substantially free of humins, wherein        the filtration temperature is sufficiently high to keep the FDCA        soluble in the solvent.

In still further embodiments, the process comprises:

-   -   i) oxidizing a feedstock comprising HMF and humins in a solvent        to produce a mixture comprising crude humins-containing FDCA;    -   ii) filtering the mixture under high temperature to obtain a        filtrate comprising FDCA substantially free of humins, wherein        the filtration temperature is sufficiently high to keep the FDCA        soluble in the solvent;    -   iii) esterifying the FDCA substantially free of humins with an        alcohol to produce a crude ester of FDCA; and    -   iv) purifying the crude ester of FDCA obtained in step iii) by        distillation or sublimation.

DETAILED DESCRIPTION OF THE DISCLOSURE

The disclosures of all cited patent and non-patent literature areincorporated herein by reference in their entirety.

As used herein, the term “embodiment” or “disclosure” is not meant to belimiting, but applies generally to any of the embodiments defined in theclaims or described herein. These terms are used interchangeably herein.

Unless otherwise disclosed, the terms “a” and “an” as used herein areintended to encompass one or more (i.e., at least one) of a referencedfeature.

The features and advantages of the present disclosure will be morereadily understood by those of ordinary skill in the art from readingthe following detailed description. It is to be appreciated that certainfeatures of the disclosure, which are, for clarity, described above andbelow in the context of separate embodiments, may also be provided incombination in a single element. Conversely, various features of thedisclosure that are, for brevity, described in the context of a singleembodiment, may also be provided separately or in any sub-combination.In addition, references to the singular may also include the plural (forexample, “a” and “an” may refer to one or more) unless the contextspecifically states otherwise.

The use of numerical values in the various ranges specified in thisapplication, unless expressly indicated otherwise, are stated asapproximations as though the minimum and maximum values within thestated ranges were both proceeded by the word “about”. In this manner,slight variations above and below the stated ranges can be used toachieve substantially the same results as values within the ranges.Also, the disclosure of these ranges is intended as a continuous rangeincluding each and every value between the minimum and maximum values.

As used herein:

The acronym FDCA means 2,5-furan dicarboxylic acid.

The acronym FFCA means 5-formylfuran-2-carboxylic acid.

The acronym FDME means 2,5-furan dicarboxylic acid dimethyl ester.

The acronym FDMME means the monomethyl ester of 2,5-furan dicarboxylicacid.

The acronym FFME means the methyl ester of 5-formylfuran-2-carboxylicacid.

The acronym HMF means 5-hydroxymethyl furfural.

The acronym AcMF means 5-acetoxymethyl-2-furaldehyde.

The term HMF means hydroxymethyl furfural and should also be understoodto include derivatives of HMF wherein HMF has reacted with the solventor with another HMF molecule or derivative to form directly relatedderivatives of HMF. Examples of HMF derivatives can include ethers whenthe solvent includes an alcohol and esters when the solvent includes acarboxylic acid.

The term HMF dimer refers to the product of etherification of twomolecules of HMF to form 5,5′-oxy(bismethylene)-2-furaldehyde.

The term “humins” means a highly colored, generally brown to black,amorphous or non-crystalline polymers resulting from the dehydration ofsugars. Humins are generally insoluble in water.

The phrase “substantially free of humins” means a composition thatcontains less than 100 parts per million of humins as measured by HPLCor SEC analytical methods. In other embodiments, the compositioncontains less than 75 ppm of humins, or less than 50 ppm of humins orless than 25 ppm humins or less than 20 ppm humins or less than 15 ppmhumins or less than 10 ppm humins, as measured by HPLC/SEC.

The phrase “ester of FDCA” means a composition comprising greater than50 percent by weight of the diester of 2,5-furandicarboxylic acid, basedon the total weight of the ester of FDCA. The remainder of thecomposition can be the monoester of 2,5-furandicarboxylic acid, theester of 5-formylfuran-2-carboxylate, 5-formylfuran-2-carboxylic acid,2,5-furandicarboxylic acid or a combination thereof. In otherembodiments, the diester can comprise greater than 90 percent or greaterthan 95 percent or greater than 96 percent or greater than 97 percent orgreater than 98 percent or greater than 99 percent by weight of theester of FDCA, with the other furan compounds making up the remainder ofthe composition. Furthermore, the percentage by weight is based on thedry composition, for example, a composition dried for at least 8 hoursin a vacuum oven at a temperature in the range of from 40° C. to 100° C.and at a pressure of less than or equal to 0.5 bar.

The phrase “at a temperature sufficiently high to keep the FDCA solublein the solvent” means a temperature in the range of from 50° C. to 275°C., so that at least 95 percent by weight of the FDCA is dissolved inthe solvent, based on the total amount of FDCA in the mixture.

The phrase “alcohol source” means a molecule which, in the presence ofwater and optionally an acid, forms an alcohol.

In some embodiments, the disclosure relates to a process comprising:

-   -   a) oxidizing a feedstock comprising HMF and humins to produce a        mixture comprising crude humins-containing FDCA;    -   b) separating the mixture to obtain a solid FDCA/humins        composition;    -   c) esterifying the solid FDCA/humins composition to produce a        crude ester of FDCA; and    -   d) purifying the crude ester of FDCA obtained in step c) by        distillation or by sublimation to produce a purified ester of        FDCA substantially free of humins.

In other embodiments, the process comprises:

-   -   i) oxidizing a feedstock comprising HMF and humins in a solvent        to produce a mixture comprising crude humins-containing FDCA        mixture; and    -   ii) filtering the mixture under high temperature to obtain a        filtrate comprising FDCA substantially free from humins, wherein        the filtration temperature is sufficiently high to keep the FDCA        soluble in the solvent.

In still further embodiments, the process comprises:

-   -   i) oxidizing a feedstock comprising HMF and humins in a solvent        to produce a mixture comprising crude humins-containing FDCA        mixture;    -   ii) filtering the mixture under high temperature to obtain a        filtrate comprising FDCA substantially free from humins, wherein        the filtration temperature is sufficiently high to keep the FDCA        soluble in the solvent;    -   iii) esterifying the FDCA substantially free of humins to        produce a crude ester of FDCA; and    -   iv) purifying the crude ester of FDCA obtained in step c) by        distillation or sublimation.

The feedstock comprising HMF and humins can be produced by thedehydration of hexose sugars, starch, amylose, galactose, cellulose,hemicellulose, inulin, fructan, glucose, fructose, sucrose, maltose,cellobiose, lactose, and/or sugar oligomers. Depending upon thedehydration process conditions, the feedstock can comprise HMF andhumins, optionally further comprising one or more of an HMF ether or anHMF ester, for example, 5-acetoxymethyl-2-furaldehyde. The amount ofhumins in the feedstock can vary depending upon the process conditionsused to form the HMF. In some embodiments, the amount of humins in thefeedstock comprising HMF and humins can be in the range of from greaterthan or equal to 10 parts per million (ppm) to up to 500,000 ppm, basedon the total weight of HMF and humins in the feedstock. In otherembodiments, the amounts of humins can be in the range of from 100 ppmto 500,000 ppm or from 2500 ppm to 250,000 ppm or from 10,000 to 200,000ppm or from 50,000 to 200,000 ppm, based on the total weight of HMF andhumins.

The oxidation step a) can be conducted by contacting a feedstockcomprising 5-hydroxymethyl furfural (HMF) and humins with an oxidant inthe presence of an oxidation catalyst to produce a crudehumins-containing FDCA. The oxidation step is typically carried out in asolvent, for example, acetic acid or a mixture of acetic acid and water.The oxidation catalyst can be a homogeneous oxidation catalyst.

Any suitable homogeneous oxidation catalyst which is effective foroxidizing HMF, HMF esters, or HMF ethers to FDCA and/or derivatives ofFDCA can be used. The homogeneous oxidation catalysts can include, forexample, metal catalysts comprising one or more transition metals. Insome embodiments, the metal catalyst comprises cobalt, manganese or acombination thereof. In other embodiments, the metal catalyst furthercomprises zirconium or cerium. The oxidation catalyst may furtherinclude bromine. In some embodiments, the metal catalyst may react withthe bromine and form in-situ metal bromides. In some embodiments, themetal catalyst comprises or consists essentially of from 59 to 5900parts per million of Co, from 55 to 5500 parts per million of Mn, andfrom 203 to 20000 parts per million of Br. All of the parts per millionof the catalyst are based on the total weight of the oxidation reactionmixture. Still other metals have previously been found useful forcombining with Co/Mn/Br, for example, Zr and/or Ce and may be included.

Each of the metal components can be provided in any of their known ionicforms. Preferably the metal or metals are in a form that is soluble inthe oxidation solvent. Examples of suitable counterions for cobalt andmanganese include, but are not limited to, carbonate, acetate, acetatetetrahydrate and halide. In some embodiments, the bromine can be in theform of the bromide, for example, hydrogen bromide, sodium bromide,ammonium bromide or potassium bromide. In some embodiments, cobaltacetate, cobalt acetate tetrahydrate, manganese acetate and/or manganeseacetate tetrahydrate can be used.

The oxidation step can be performed at a temperature in the range offrom 120° C. to 250° C. In other embodiments, the oxidation step can beperformed at a temperature in the range of from 125° C. to 250° C. or130° C. to 240° C. The oxidation step further comprises an oxidant, forexample, oxygen gas or an oxygen-containing gas. As an oxygen-containinggas, air or a mixture of oxygen and nitrogen can be used. In someembodiments, the pressure of the oxidant in step a) is such that anoxygen partial pressure of from 0.2 to 100 bar is provided. In otherembodiments, the oxygen partial pressure can be in the range of from 0.2to 50 bar or from 0.2 to 30 bar or from 0.2 to 21 bar.

The oxidation step a) produces a mixture comprising crudehumins-containing FDCA. Step b) comprises separating the mixture toobtain a solid FDCA/humins composition. The solid FDCA/huminscomposition can be separated by filtration or by centrifugation. Theseparation can be conducted at a temperature at or below the temperatureof the oxidation temperature of step a). In some embodiments, theseparation step is conducted at a temperature below the oxidationtemperature, for example, a temperature in the range of from 20° C. to200° C. In other embodiments, the separation temperature is in the rangeof from 30° C. to 175° C. or from 40° C. to 150° C. The oxidationmixture comprising the crude humins-containing FDCA from step a) can becooled in the oxidation vessel, in a separate vessel or in a series ofvessels that gradually cool the reaction temperature to the desiredseparation temperature. In some embodiments, the crude humins-containingFDCA is cooled via evaporative cooling via a series of evaporativecooling vessels.

Separation of the mixture yields a solid FDCA/humins composition and amother liquor composition. In some embodiments, the solid FDCA/huminscomposition comprises in the range of from greater than or equal to 10ppm to 100,000 ppm of humins, based on the total weight of the FDCA andthe humins. In other embodiments, the solids FDCA/humins compositioncomprises in the range of from 100 ppm to 50,000 ppm, or from 500 ppm to25,000 ppm, or from 1,000 to 20,000 ppm, or from 10,000 to 20,000 ppm ofhumins, based on the total weight of FDCA and humins. The solidFDCA/humins composition obtained from the separation step can be used asis, without a further purification step.

In step c), the solid FDCA/humins composition obtained in step b) can beesterified to produce a crude ester of FDCA. The solid FDCA/humins canbe used, as is from step b), and can be in the form of a dry solid or awet cake. The esterification can be accomplished by heating the solidsFDCA/humins composition with an excess of an alcohol having in the rangeof from 1 to 12 carbon atoms, especially alkyl alcohols. Suitablealcohols can include, for example methanol, ethanol, propanol, butanol,pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol,dodecanol or isomers thereof. In some embodiments, the alcohol is in therange of from 1 to 6 carbon atoms or in the range of from 1 to 4 carbonatoms or in the range of from 1 to 2 carbon atoms. In some embodiments,the alcohol is methanol and the ester of FDCA is FDME.

In some embodiments, the percentage of FDCA and alcohol that can be fedto the reactor can be expressed as a weight percentage of the FDCA basedon the total amount of FDCA and the alcohol. For example, the weight ofFDCA can be in the range of from 1 to 70 percent by weight, based on thetotal weight of the FDCA and the alcohol. Correspondingly, the alcoholcan be present at a weight percentage of about 30 to 99 percent byweight, based on the total amount of FDCA and the alcohol. In otherembodiments, the FDCA can be present in the range of from 2 to 60percent, or from 5 to 50 percent, or from 10 to 50 percent, or from 15to 50 percent, or from 20 to 50 percent by weight, wherein allpercentages by weight are based on the total amount of FDCA and thealcohol.

In other embodiments, the ratio of alcohol to the FDCA can be expressedin a molar ratio wherein the molar ratio of the alcohol to the FDCA canbe in the range of from 2.01:1 to 40:1. In other embodiments, the molarratio of the alcohol to FDCA can be in the range of from 2.2:1 to 40:1,or 2.5:1 to 40:1, or 3:1 to 40:1, or 4:1 to 40:1, or 8:1 to 40:1, or10:1 to 40:1, or 15:1 to 40:1, or 20:1 to 40:1, or 25:1 to 40:1, or 30:1to 40:1.

At least a portion of the alcohol can be replaced with an alcoholsource. The alcohol source is a molecule which, in the presence of waterand optionally an acid forms an alcohol. In some embodiments, thealcohol source is an acetal, an orthoformate, an alkyl carbonate, atrialkyl borate, a cyclic ether comprising 3 or 4 atoms in the ring or acombination thereof. Suitable acetals can include, for example, dialkylacetals, wherein the alkyl portion of the acetal comprises in the rangeof from 1 to 12 carbon atoms. In some embodiments, the acetal can be1,1-dimethoxyethane (acetaldehyde dimethyl acetal), 2,2 dimethoxypropane(acetone dimethyl acetal), 1,1-diethoxyethane (acetaldehyde diethylacetal) or 2,2 diethoxypropane (acetone diethyl acetal). Suitableorthoformates can be, for example, trialkyl orthoformate wherein thealkyl group comprises in the range of from 1 to 12 carbon atoms. In someembodiments, the orthoester is trimethyl orthoformate or triethylorthoformate. Suitable alkyl carbonates can be dialkyl carbonateswherein the alkyl portion comprises in the range of from 1 to 12 carbonatoms. In some embodiments, the dialkyl carbonate is dimethyl carbonateor diethyl carbonate. Suitable trialkyl borates can be, for example,trialkyl borates wherein the alkyl portion comprises in the range offrom 1 to 12 carbon atoms. In some embodiments, the trialkyl borate istrimethyl borate or triethyl borate. A cyclic ether can also be usedwherein the cyclic ether has 3 or 4 carbon atoms in the ring. In someembodiments, the cyclic ether is ethylene oxide or oxetane. If used, thealcohol source can be used to replace in the range of from 0.1 percentto 100 percent of the alcohol, based on the total weight of the alcoholin the process.

In further embodiments, combinations of the alcohol and the alcoholsource can also be used. In some embodiments, the percentage by weightof the alcohol can be in the range of from 0.001 percent to 99.999percent by weight, based on the total weight of the alcohol and thealcohol source. In other embodiments, the alcohol can be present at apercentage by weight in the range of from 1 to 99 percent, or from 5 to95 percent, or from 10 to 90 percent, or from 20 to 80 percent, or from30 to 70 percent, or from 40 to 60 percent, wherein the percentages byweight are based on the total weight of the alcohol and the alcoholsource.

The solid FDCA/humins mixture is fed to a reactor and is contacted withan excess of alcohol, optionally with an esterification catalyst. Theesterification reaction can be conducted at elevated temperatures, forexample, in the range of from 50° C. to 325° C. and at a pressure in therange of from 1 bar and 140 bar for a sufficient time to produce thedesired ester of FDCA. In other embodiments, the temperature can be inthe range of from 75° C. to 325° C., or from 100° C. to 325° C., or from125° C. to 325° C., or from 150° C. to 320° C., or from 160° C. to 315°C., or from 170 to 310° C. In other embodiments, the temperature can bein the range of from 50° C. to 150° C., or from 65° C. to 140° C., orfrom 75° C. to 130° C. In still further embodiments, the temperature canbe in the range of from 250° C. to 325° C., or from 260° C. to 320° C.,or from 270° C. to 315° C., or from 275° C. to 310° C., or from 280° C.to 310° C. In some embodiments, the pressure can be in the range of from5 bar to 130 bar, or from 15 bar to 120 bar, or from 20 bar to 120 bar.In other embodiments, the pressure can be in the range of from 1 bar to5 bar, or 1 bar to 10 bar, or 1 bar to 20 bar. The pressure andtemperature of the step c) are chosen so that contents of the reactorcomprise a liquid phase and at least a portion of the contents are inthe gas phase.

Step c) can optionally be conducted in the presence of an esterificationcatalyst, for example, the catalyst can be cobalt (II) acetate, iron(II) chloride, iron (III) chloride, iron (II) sulfate, iron (III)sulfate, iron (II) nitrate, iron (III) nitrate, iron (II) oxide, iron(III) oxide, iron (II) sulfide, iron (III) sulfide, iron (II) acetate,iron (III) acetate, magnesium (II) acetate, magnesium (II) hydroxide,manganese (II) acetate, phosphoric acid, sulfuric acid, zinc (II)acetate, zinc stearate, a solid acid catalyst, a zeolite solid catalyst,or a combination thereof. The metal acetates, chlorides and hydroxidescan be used as the hydrated salts. In some embodiments, the catalyst canbe cobalt (II) acetate, iron (II) chloride, iron (III) chloride,magnesium (II) acetate, magnesium hydroxide, zinc (II) acetate, or ahydrate thereof. In still further embodiments, the catalyst can be iron(II) chloride, iron (III) chloride, or a combination thereof. In otherembodiments, the catalyst can be cobalt acetate. In some embodiments,the catalyst can be sulfuric acid, hydrobromic acid, hydrochloric acid,boric acid, or another suitable Brønsted acid. Combinations of any ofthe above catalysts may also be useful. If present, a catalyst can beused at a rate of 0.01 to 5.0 percent by weight, based on the totalweight of the FDCA, alcohol and optionally the alcohol source, and thecatalyst. In other embodiments, the amount of catalyst present can be inthe range of from 0.2 to 4.0, or from 0.5 to 3.0, or from 0.75 to 2.0,or from 1.0 to 1.5 percent by weight, wherein the percentages by weightare based on the total amount of FDCA, methanol and the catalyst.

The catalyst can also be a solid acid catalyst having the thermalstability required to survive reaction conditions. The solid acidcatalyst may be supported on at least one catalyst support. Examples ofsuitable solid acids include without limitation the followingcategories: 1) heterogeneous heteropolyacids (HPAs) and their salts, 2)natural or synthetic minerals (including both clays and zeolites), suchas those containing alumina and/or silica, 3) cation exchange resins, 4)metal oxides, 5) mixed metal oxides, 6) metal salts such as metalsulfides, metal sulfates, metal sulfonates, metal nitrates, metalphosphates, metal phosphonates, metal molybdates, metal tungstates,metal borates or combinations thereof. The metal components ofcategories 4 to 6 may be selected from elements from Groups 1 through 12of the Periodic Table of the Elements, as well as aluminum, chromium,tin, titanium, and zirconium. Examples include, without limitation,sulfated zirconia and sulfated titania.

Suitable HPAs include compounds of the general formula X_(a)M_(b)O_(c)^(q−), where X is a heteroatom such as phosphorus, silicon, boron,aluminum, germanium, titanium, zirconium, cerium, cobalt or chromium, Mis at least one transition metal such as tungsten, molybdenum, niobium,vanadium, or tantalum, and q, a, b, and c are individually selectedwhole numbers or fractions thereof. Nonlimiting examples of salts ofHPAs include, for example, lithium, sodium, potassium, cesium,magnesium, barium, copper, gold and gallium, and ammonium salts.Examples of HPAs suitable for the disclosed process include, but are notlimited to, tungstosilicic acid (H₄[SiW₁₂O₄₀].xH₂O), tungstophosphoricacid (H₃[PW₁₂O₄₀].xH₂O), molybdophosphoric acid (H₃[PMo₁₂O₄₀].xH₂O),molybdosilicic acid (H₄[SiMo₁₂O₄₀].xH₂O), vanadotungstosilicic acid(H_(4+n)[SiV_(n)W_(12−n)O₄₀].xH₂O), vanadotungstophosphoric acid(H_(3+n)[PV_(n)W_(12−n)O₄₀].xH₂O), vanadomolybdophosphoric acid(H_(3+n)[PV_(n)Mo_(12−n)O₄₀].xH₂O), vanadomolybdosilicic acid(H_(4+n)[SiV_(n)Mo_(12−n)O₄₀].xH₂O), molybdotungstosilicic acid(H₄[SiMo_(n)W_(12−n)O₄₀].xH₂O), molybdotungstophosphoric acid(H₃[PMo_(n)W_(12−n)O₄₀].xH₂O), wherein n in the formulas is an integerfrom 1 to 11 and x is an integer of 1 or more.

Natural clay minerals are well known in the art and include, withoutlimitation, kaolinite, bentonite, attapulgite, and montmorillonite.

In an embodiment, the solid acid catalyst is a cation exchange resinthat is a sulfonic acid functionalized polymer. Suitable cation exchangeresins include, but are not limited to the following: styrenedivinylbenzene copolymer-based strong cation exchange resins such asAMBERLYS™ and DOWEX® available from Dow Chemicals (Midland, Mich.) (forexample, DOWEX® Monosphere M-31, AMBERLYST™ 15, AMBERLITE™ 120); CGresins available from Resintech, Inc. (West Berlin, N.J.); Lewatitresins such as MONOPLUS™ S 100H available from Sybron Chemicals Inc.(Birmingham, N.J.); fluorinated sulfonic acid polymers (these acids arepartially or totally fluorinated hydrocarbon polymers containing pendantsulfonic acid groups, which may be partially or totally converted to thesalt form) such as NAFION® perfluorinated sulfonic acid polymer, NAFION®Super Acid Catalyst (a bead-form strongly acidic resin which is acopolymer of tetrafluoroethylene andperfluoro-3,6-dioxa-4-methyl-7-octene sulfonyl fluoride, converted toeither the proton (H⁺), or the metal salt form) available from DuPontCompany (Wilmington, Del.).

In an embodiment, the solid acid catalyst is a supported acid catalyst.The support for the solid acid catalyst can be any solid substance thatis inert under the reaction conditions including, but not limited to,oxides such as silica, alumina, titania, sulfated titania, and compoundsthereof and combinations thereof; barium sulfate; calcium carbonate;zirconia; carbons, particularly acid washed carbon; and combinationsthereof. Acid washed carbon is a carbon that has been washed with anacid, such as nitric acid, sulfuric acid or acetic acid, to removeimpurities. The support can be in the form of powder, granules, pellets,or the like. The supported acid catalyst can be prepared by depositingthe acid catalyst on the support by any number of methods well known tothose skilled in the art of catalysis, such as spraying, soaking orphysical mixing, followed by drying, calcination, and if necessary,activation through methods such as reduction or oxidation. The loadingof the at least one acid catalyst on the at least one support is in therange of 0.1-20 weight percent based on the combined weights of the atleast one acid catalyst and the at least one support. Certain acidcatalysts perform better at low loadings such as 0.1-5%, whereas otheracid catalysts are more likely to be useful at higher loadings such as10-20%. In an embodiment, the acid catalyst is an unsupported catalysthaving 100% acid catalyst with no support such as, pure zeolites andacidic ion exchange resins.

Examples of supported solid acid catalysts include, but are not limitedto, phosphoric acid on silica, NAFION®, a sulfonated perfluorinatedpolymer, HPAs on silica, sulfated zirconia, and sulfated titania. In thecase of NAFION® on silica, a loading of 12.5% is typical of commercialexamples.

In another embodiment, the solid acid catalyst comprises a sulfonateddivinylbenzene/styrene copolymer, such as AMBERLYST™ 70.

In one embodiment, the solid acid catalyst comprises a sulfonatedperfluorinated polymer, such as NAFION® supported on silica (SiO₂).

In one embodiment, the solid acid catalyst comprises natural orsynthetic minerals (including both clays and zeolites), such as thosecontaining alumina and/or silica.

Zeolites suitable for use herein can be generally represented by thefollowing formula M_(2/n)O.Al₂O₃.xSiO₂.yH₂O wherein M is a cation ofvalence n, x is greater than or equal to about 2, and y is a numberdetermined by the porosity and the hydration state of the zeolite,generally from about 2 to about 8. In naturally occurring zeolites, M isprincipally represented by Na, Ca, K, Mg and Ba in proportions usuallyreflecting their approximate geochemical abundance. The cations M areloosely bound to the structure and can frequently be completely orpartially replaced with other cations by conventional ion exchange.

The zeolite framework structure has corner-linked tetrahedra with Al orSi atoms at centers of the tetrahedra and oxygen atoms at the corners.Such tetrahedra are combined in a well-defined repeating structurecomprising various combinations of 4-, 6-, 8-, 10-, and 12-memberedrings. The resulting framework structure is a pore network of regularchannels and cages that is useful for separation. Pore dimensions aredetermined by the geometry of the aluminosilicate tetrahedra forming thezeolite channels or cages, with nominal openings of about 0.26 nm for6-member rings, about 0.40 nm for 8-member rings, about 0.55 nm for10-member rings, and about 0.74 nm for 12-member rings (these numbersassume the ionic radii for oxygen). Zeolites with the largest pores,being 8-member rings, 10-member rings, and 12-member rings, arefrequently considered small, medium and large pore zeolites,respectively.

In a zeolite, the term “silicon to aluminum ratio” or, equivalently,“Si/Al ratio” means the ratio of silicon atoms to aluminum atoms. Poredimensions are critical to the performance of these materials incatalytic and separation applications, since this characteristicdetermines whether molecules of certain size can enter and exit thezeolite framework.

In practice, it has been observed that very slight decreases in ringdimensions can effectively hinder or block movement of particularmolecular species through the zeolite structure. The effective poredimensions that control access to the interior of the zeolites aredetermined not only by the geometric dimensions of the tetrahedraforming the pore opening, but also by the presence or absence of ions inor near the pore. For example, in the case of zeolite type A, access canbe restricted by monovalent ions, such as Na⁺ or K⁺, which are situatedin or near 8-member ring openings as well as 6-member ring openings.Access can be enhanced by divalent ions, such as Ca²⁺, which aresituated only in or near 6-member ring openings. Thus, the potassium andsodium salts of zeolite A exhibit effective pore openings of about 0.3nm and about 0.4 nm respectively, whereas the calcium salt of zeolite Ahas an effective pore opening of about 0.5 nm.

The presence or absence of ions in or near the pores, channels and/orcages can also significantly modify the accessible pore volume of thezeolite for sorbing materials. Representative examples of zeolites are(i) small pore zeolites such as NaA (LTA), CaA (LTA), Erionite (ERI),Rho (RHO), ZK-5 (KFI) and chabazite (CHA); (ii) medium pore zeolitessuch as ZSM-5 (MFI), ZSM-11 (MEL), ZSM-22 (TON), and ZSM-48 (*MRE); and(iii) large pore zeolites such as zeolite beta (BEA), faujasite (FAU),mordenite (MOR), zeolite L (LTL), NaX (FAU), NaY (FAU), DA-Y (FAU) andCaY (FAU). The letters in parentheses give the framework structure typeof the zeolite.

Zeolites suitable for use herein include medium or large pore, acidic,hydrophobic zeolites, including without limitation ZSM-5, faujasites,beta, mordenite zeolites or mixtures thereof, having a high silicon toaluminum ratio, such as in the range of 5:1 to 400:1 or 5:1 to 200:1.Medium pore zeolites have a framework structure consisting of10-membered rings with a pore size of about 0.5-0.6 nm. Large porezeolites have a framework structure consisting of 12-membered rings witha pore size of about 0.65 to about 0.75 nm. Hydrophobic zeolitesgenerally have Si/Al ratios greater than or equal to about 5, and thehydrophobicity generally increases with increasing Si/Al ratios. Othersuitable zeolites include without limitation acidic large pore zeolitessuch as H—Y with Si/Al in the range of about 2.25 to 5.

The esterification step can produce a crude ester of FDCA comprising thedesired diester of FDCA and optionally, further comprising the monoester of FDCA, the alkyl ester of 5-formylfuran-2-carboxylic acid,5-formylfuran-2-carboxylic acid, and unreacted FDCA. In someembodiments, the product of the esterification can be removed from thereactor by removing a vapor component, wherein the vapor componentcomprises water, the alcohol and the crude ester of FDCA. If the crudeester of FDCA is removed via the vapor phase, humins remain in theliquid phase and the crude ester of FDCA comprises very little if anyhumins. Other impurities may be present, but humins generally are notcontained in the vapor phase. In other embodiments, the contents of theesterification reactor can be cooled and the crude ester of FDCA can beremoved via a solid liquid separation step. The cooling step, ifpresent, can be performed in the esterification reactor, in a separatecooling vessel or in a series of separate cooling vessels, wherein eachsuccessive vessel further cools the mixture when compared to theprevious cooling vessel. The cooling of the contents of theesterification step c) can cause the ester of FDCA to crystallize. Ifthe crude ester of FDCA is crystallized, then humins will be present inthe solids phase. The crude ester of FDCA can then be separated byfiltration or centrifugation. Separation of the solids comprising thecrude ester of FDCA also yields a mother liquor. The mother liquor cancomprise the alcohol and water. If an alcohol source is used, then themother liquor can also comprise one or more of the by-products from thehydrolysis of the alcohol source. For example, in the presence of water,trimethyl orthoformate is known to form methanol and methyl formate.Other hydrolysis products of the disclosed alcohol sources arewell-known in the art and can be present in the mother liquor.

The process further comprises a step d), purifying the crude ester ofFDCA obtained in step c) by crystallization, distillation or sublimationto produce a purified ester of FDCA substantially free of humins. Insome embodiments, the purification step d) is a distillation step,wherein the distillation is performed at low pressure, for example, inthe range of from less than 1 bar to 0.0001 bar. In other embodiments,the pressure can be in the range of from 0.75 bar to 0.001 bar or from0.5 bar to 0.01 bar. In other embodiments, the purification step d) is asublimation step wherein the solid crude ester of FDCA is sublimed toprovide the purified ester of FDCA substantially free of humins.

In some embodiments, the amount of humins in the purified ester of FDCAis less than 100 ppm as determined by size exclusion chromatography. Instill further embodiments, the purified ester of FDCA has a b* colorvalue of less than 3, or less than 2, as determined by LAB colormeasurement. In other embodiments, the purified ester of FDCA comprisesgreater than or equal to 99% by weight of the ester of FDCA, wherein thepercentage by weight is based on the total amount of a sample of thedried ester of FDCA.

In other embodiments, the disclosure relates to a process comprising:

-   -   i) oxidizing a feedstock comprising HMF and humins in a solvent        to produce a mixture comprising crude humins-containing FDCA;        and    -   ii) filtering the mixture under high temperature to obtain a        filtrate comprising FDCA substantially free of humins, wherein        the filtration temperature is sufficiently high to keep the FDCA        soluble in the solvent.

The oxidation step can be conducted under the same oxidation conditionsas was given above for step a), with the exception that, in someembodiments, the concentration of the FDCA in the solvent is in therange of from 0.1 percent to 15 percent by weight, based on the totalweight of the solvent. In other embodiments, the concentration of theFDCA can be in the range of from 0.1 to 12.0 percent by weight. In someembodiments, the solvent for step i) is acetic acid, or mixture ofacetic acid and water.

Following the oxidation step, the process further comprises a step ii),filtering the mixture under high temperature to obtain a filtratecomprising FDCA substantially free of humins, wherein the filtrationtemperature is sufficiently high to keep the FDCA soluble in thesolvent. Temperatures sufficiently high to keep the FDCA soluble in thesolvent are dependent on the concentration of the FDCA in the solvent,and can easily be determined. For example, in a solvent containing 93wt. % acetic acid and 7 wt. % water, FDCA is typically soluble up toabout 10 percent by weight at a temperature of between 180° C. and 275°C. At lower temperature, for example, temperatures as low as 50° C.,lesser amounts of FDCA are soluble in acetic acid or a mixture of aceticacid and water. In some embodiments, the high temperature of thefiltration step ii) can be in the range of from 50° C. to 275° C. Inother embodiments, the filtration step ii) can be conducted at atemperature in the range of from 75° C. to 250° C. or from 100° C. to225° C. or from 120° C. to 200° C. It has been found that the solubilityof the humins is dependent at least on the solvent composition, forexample, the amount of water in the acetic acid. Therefore, thefiltration step ii) can provide a filtrate wherein the filtratecomprises FDCA substantially free from humins.

In some embodiments, the filtrate comprising FDCA substantially free ofhumins can be evaporated or distilled to provide solid impure FDCAsubstantially free of humins. The FDCA substantially free of humins canbe purified by any of the known methods, for example, using at least onecrystallization step. Suitable crystallization solvents can include forexample, acetic acid, a mixture of acetic acid and water or water.

In still further embodiments, the disclosure relates to a processcomprising:

-   -   i) oxidizing a feedstock comprising HMF and humins in a solvent        to produce a mixture comprising crude humins-containing FDCA,        wherein the concentration of the FDCA in the solvent is in the        range of from 0.1 percent to 15 percent by weight, based on the        total weight of the solvent;    -   ii) filtering the mixture under high temperature to obtain a        filtrate comprising FDCA substantially free of humins, wherein        the filtration temperature is sufficiently high to keep the FDCA        soluble in the solvent;    -   iii) esterifying the FDCA substantially free of humins to        produce a crude ester of FDCA; and    -   iv) purifying the crude ester of FDCA obtained in step c).

Process steps i) and ii) are identical to steps i) and ii) describedabove. The process further provides steps iii) and iv). The step iii) ofesterifying the FDCA substantially free of humins can be furthercomprises a step of removing at least a portion of the solvent toprovide solid FDCA substantially free of humins. The esterification stepiii) can utilize dry solids or a wet cake of the FDCA substantially freeof humins. The esterifying step can use the same conditions as describedabove for the esterification step c). The product of step iii) is acrude ester of FDCA. In some embodiments, the ester of FDCA is a methylester of FDCA.

The crude ester of FDCA can be purified in step iv) by crystallization,distillation or sublimation. The process steps for the sublimation areidentical to those given for the purification step d).

The described processes can provide a purified ester of FDCAsubstantially free of humins or FDCA substantially free of humins. Thepurified ester of FDCA comprises less than 100 ppm humins, a b* value ofless than 3 and greater than 99 percent by weight of the diester ofFDCA, based on the total weight of the purified ester of FDCA.

Non-limiting examples of the processes disclosed herein include:

1. A process comprising:

-   -   a) oxidizing a feedstock comprising HMF and humins to produce a        mixture comprising crude humins-containing FDCA;    -   b) separating the mixture to obtain a solid FDCA/humins        composition;    -   c) esterifying the solid FDCA/humins composition with an alcohol        to produce a crude ester of FDCA; and    -   d) purifying the crude ester of FDCA obtained in step c) to        produce a purified ester of FDCA substantially free of humins.        2. The process of embodiment 1 wherein the purified ester of        FDCA is 2,5-furan dicarboxylic acid dimethyl ester.        3. A process comprising:    -   i) oxidizing a feedstock comprising HMF and humins in a solvent        to produce a mixture comprising crude humins-containing FDCA;        and    -   ii) filtering the mixture under high temperature to obtain a        filtrate comprising FDCA substantially free of humins, wherein        the filtration temperature is sufficiently high to keep the FDCA        soluble in the solvent.        4. The process of embodiment 3 wherein the high temperature of        the filtration step ii) is in the range of from 50° C. to 275°        C.        5. The process of embodiment 3, further comprising a step iii)        crystallizing the 2,5-furan dicarboxylic acid substantially free        of humins to obtain further purified 2,5-furan dicarboxylic        acid.        6. The process of embodiment 3 wherein the solvent is acetic        acid or a mixture of acetic acid and water.        7. A process comprising:    -   i) oxidizing a feedstock comprising HMF and humins in a solvent        to produce a mixture comprising crude humins-containing FDCA;    -   ii) filtering the mixture under high temperature to obtain a        filtrate comprising FDCA substantially free of humins, wherein        the filtration temperature is sufficiently high to keep the FDCA        soluble in the solvent;    -   iii) esterifying the FDCA substantially free of humins with an        alcohol to produce a crude ester of FDCA; and    -   iv) purifying the crude ester of FDCA obtained in step iii) by        distillation or sublimation to obtain a purified ester.        8. The process of embodiment 7 wherein the ester of FDCA is a        methyl ester of FDCA.        9. The process of embodiment 7 wherein the high temperature of        the filtration step ii) is in the range of from 50° C. to 275°        C.        10. The process of embodiment 7 wherein the solvent is acetic        acid or acetic acid and water.        11. The process of any one of embodiments 1, 3 or 7 wherein the        purified ester of FDCA comprises less than 100 ppm humins as        determined by size exclusion chromatography, a b* value of less        than 3 as determined by LAB color measurement, and greater than        99 percent by weight of the diester of FDCA, based on the total        weight of the purified ester of FDCA.        12. The process of any one of embodiments 1 or 7, wherein at        least a portion of the alcohol is replaced with an alcohol        source.

EXAMPLES

Unless otherwise noted, all chemicals and reagents are available fromthe Sigma-Aldrich Company, St. Louis, Mo.

ACS grade glacial acetic acid was obtained from Fisher Scientific.

The HMF feed was provided by Archer Daniels Midland (ADM) Company,Decatur, Ill.

Methanol was obtained from EMD Millipore (catalog #MX-0472-6).

Acetonitrile was obtained from Fisher Scientific (catalog #A955-1).

Isopropanol was also obtained from Fisher Scientific (catalog #A4641L1).

The following abbreviations are used in the examples: “° C.” meansdegrees Celsius; “wt %” means weight percent; “g” means gram; “min”means minute(s); “μL” means microliter; “ppm” means microgram per gram,“μm” means micrometer; “mL” means milliliter; “mm” means millimeter and“mL/min” means milliliter per minute; “slpm” means standard liters perminute; “HMF” means 5-(hydroxymethyl)furfural, “AcMF” means5-(acetoxymethyl)furfural, “DMF” means dimethylformamide, “FFCA” means5-formyl-2-furancarboxylic acid, “FDCA” means 2,5-furandicarboxylicacid, “FDME” means dimethyl-furan-2,5-dicarboxylate, “FFME” means formylfuran methyl ester, “FDMME” means furan dicarboxylic acid mono methylester.

Test/General Methods HPLC Analysis

HPLC analysis was used as one means to measure the FDCA, FDMME & FDMEcontents of the product mixture. An Agilent 1200 series HPLC equippedwith a ZORBAX™ SB-Aq column (4.6 mm×250 mm, 5 μm) and photodiode arraydetector was used for the analysis of the reaction samples. Thewavelength used to monitor the reaction was 280 nm. The HPLC separationof FDME, FDCA and FDMME was achieved using a gradient method with a 1.0mL/min flow rate combining two mobile phases: Mobile Phase A: 0.5% v/vtrifluoroacetic acid (TFA) in water and Mobile Phase B: acetonitrile.The column was held at 60° C. and 2 μL injections of samples wereperformed. Analyzed samples were diluted to <0.1 wt % for components ofinterest in a 50:50 (v/v) acetonitrile/isopropanol solvent. The solventcomposition and flow rates used for the gradient method is given inTable 1 with linear changes occurring over the corresponding stepwhenever the composition changes.

TABLE 1 Gradient program for HPLC Start Volume % Mobile Volume % Mobiletime Phase B, at Phase B, at End of Step (min) Beginning of Step Step 10.0 0 0 2 6.0 0 80 3 20.0 80 80 4 25.0 80 0 5 25.1 0 0 6 30.0 0 0

Retention times were obtained by injecting analytical standards of eachcomponent onto the HPLC. The amount of the analyte in weight percent wastypically determined by injection of two or more injections from a givenprepared solution and averaging the area measured for the componentusing the OpenLAB CDS C.01.05 software. The solution analyzed by HPLCwas generated by dilution of a measured mass of the reaction sample witha quantified mass of 50:50 (v/v) acetonitrile/isopropanol solvent.Quantification was performed by comparing the areas determined in theOpenLAB software to a linear external calibration curve at five or morestarting material concentrations. Typical R² values for the fit of suchlinear calibration curves was in excess of 0.9997.

While the presented HPLC method was used for this analysis, it should beunderstood that any HPLC method that can discriminate between products,starting materials, intermediates, impurities, and solvent can be usedfor this analysis. It should also be understood that while HPLC was usedas a method of analysis in this work, other techniques such as gaschromatography could also be optionally used for quantification whenemploying appropriate derivatization and calibration as necessary.

LAB Color Measurements

A Hunterlab COLORQUEST™ Spectrocolorimeter (Reston, Va.) was used tomeasure the color. Color numbers are measured as APHA values(Platinum-Cobalt System) according to ASTM D-1209. The “b*” color ofFDCA and/or FDME solids was calculated from the UV/VIS spectra andcomputed by the instrument. Color is commonly expressed in terms ofHunter numbers which correspond to the lightness or darkness (“L”) of asample, the color value (“a*”) on a red-green scale, and the color value(“b*”) on a yellow-blue scale. Each sample was prepared by adding 6 wt.% solids in dimethylformamide (Sigma Aldrich).

Size Exclusion Chromatography (SEC) Method for Humins Analysis

A screening assay to estimate weight concentration of soluble huminbyproduct was developed using Size Exclusion Chromatography (SEC). AnAlliance 2695 chromatograph (Waters Corporation, Milford, Mass.) wascoupled to a 2498 dual-channel UV/Visible detector (Waters Corporation).UV absorbance was collected at wavelengths of 280 and 450 nm. Thestationary phase consisting of a 4 column set (SHODEX™ KD-801, KD-802,and two KD-806M) was kept at a constant temperature of 50° C.Dimethylacetamide (Thermo Fisher, Wilmington, Del.) with 0.5% (w/v)lithium chloride (Sigma Aldrich) was used as mobile phase at a flow rateof 1 mL/min. Samples were prepared by dissolving or diluting in mobilephase, followed by agitation at room temperature for 4 hours, filteringusing 0.45 μm PTFE (Pall, Fort Washington, N.Y.), and finally injecting100 μL. A calibration curve was constructed using humin byproductisolated from an acid-catalyzed fructose dehydration reaction. Resultinghumin concentration in research samples was determined by integratingany eluting peak (450 nm absorbance) in the humin region of thechromatogram and comparing peak area to the calibration curve. A lowerlimit of detection in the sample as prepared was found to beapproximately 50 ng humins.

Example 1: Purification of FDCA

Step 1.1 Oxidation of a Feedstock Comprising HMF and Humins

Oxidation of HMF to FDCA was carried out in a 1 L titanium reactor(Autoclave Engineers, Serial #85-00534-1). The reactor was charged with440 mL of acetic acid, 23 mL of water, 4.566 g of cobalt(II) acetatetetrahydrate, 0.285 g of manganese(II) acetate tetrahydrate, and 632 μLof hydrobromic acid (48%). The reactor was pressurized to 450 psig underan air atmosphere, and heated to a temperature of 200° C. Air was thensparged in through a dip tube which had eight 229 μm diameter holes at arate of 2.0 slpm when reactor temperature reached 190° C. Nitrogen wasfed to the inlet of the condenser at 4.5 slpm. When desired temperaturewas achieved, the HMF feed was pumped into the reactor at a rate of 0.9mL/min through another dip tube which was positioned close to thereactor impeller. The composition of the HMF feed was 9.86 wt. % HMF,19.0 wt. % AcMF, 0.22 wt. % HMF dimer and 8.36 wt. % humins. Thisaddition was performed over a 45 min period. When the addition wascomplete, the reaction was further heated for an additional 50 min ofpost oxidation when only air was fed to the reactor. After postoxidation, the reactor was cooled down to room temperature anddepressurized. The FDCA solids were then vacuum filtered and dried in avacuum oven at 75° C. at a pressure of 200-300 torr. After drying, 8.8 gof crude FDCA (with a molar FDCA yield of 59.18%) solids was obtained. Adetailed analysis of this sample is given in Table 2. This crude FDCAsample is referred as sample 1.1.

Step 1.2: Esterification of FDCA Containing Humins to FDME

FDCA sample 1.1 was esterified in a 75 mL mini Parr reactor model 5050equipped with an IKA RCT Basic hotplate stirrer. A total of 6 g of FDCA(sample 1.1), 24 g methanol and TFE stir bar were added to the reactor.The reactor was placed in an aluminum block and was kept insulated. Thereactor was then purged a minimum of 5 times with nitrogen. At roomtemperature, 300 psi of N₂ was introduced in the reactor head. Thereactor was then heated to a temperature of 200° C. and both thetemperature and pressure were monitored. After 4 hr, heat was turned offand the reactor was allowed to cool to room temperature. The pressurewas then released and the reactor was opened. The reactor contents(containing mainly FDME product) were removed and transferred to analuminum pan. Solids were dried overnight on aluminum pan (whilemethanol was evaporated). The dried solids were then analyzed using HPLCand SEC. This sample of the crude methyl ester of FDCA was referred asSample 1.2 in Table 2.

Step 1.3: Purification of the Crude Methyl Ester of FDAC ViaDistillation/Sublimation

The solid product obtained after the esterification reaction waspurified using sublimation. The sublimation device was a two piece glassunit that was connected together by a metal clamp. The bottom piece ofthe sublimator had a rounded bottom so that there was a larger area forheat transfer from a heating mantle. The top piece was placed on top ofthe bottom piece with an o-ring in between so that a seal between thetop and bottom was created. The top piece of the device had a conicalwater jacket that was used to cool the sublimated vapor phase, and allowthe solids to collect on the inside of the cone. The top of the conicalpiece had a glass valve which allowed the device to be connected to avacuum source. Sublimation conditions were maintained at a pressure offrom 15 to 30 torr and a temperature of about 100° C. After completionof the sublimation, the apparatus was disassembled and the sublimationproduct was collected from the inside of the cone on the top piece. Thesublimation bottoms were collected off of the rounded bottom of thebottom piece. The solids collected from top of the sublimator wereanalyzed for their purity using HPLC, SEC and colorimetry. This samplewas referred as Sample 1.3-1 in Table 2. The sublimator was operated insuch a way that about 10% of the starting material was purified andcollected on top. The rest 90% remained in the bottom. The bottom samplewas labeled as Sample 1.3-2.

Results

TABLE 2 Results obtained after the analysis of solids collected at theend of stability tests Wt. % Wt. % Wt. % Humins Sample L* b* FDME FDMMEFDCA (ppm) 1.1 (after 96.72 13.66 0.00 0.00 96.84 1403 oxidation) 1.2(after 48.98 80.66 88.79 12.84 0.356 1045 esterification 1.3-1 (after99.99 0.19 99.248 0.752 0 <10 sublimation, collected from top) 1.3-2(after 42.46 72.65 85.273 14.389 0.338 1200 sublimation, collected frombottom)

From the results shown in Table 2, it has been surprisingly found thathumins-containing FDCA can be converted into FDME which can be furtherpurified via sublimation. The FDME obtained at the end of thesublimation did not contain any humins and also had L*>99 and b*<1. Thishigh purity monomer can be used in the manufacture of differentpolymers. Thus, the above example shows that crude humins-containingFDCA can be purified using esterification followed by sublimation toobtain a high purity polymer-grade monomer without a need forintermediate purification steps such as crystallization, hydrogenation,etc.

Example 2: Purification of FDCA without a Post Oxidation Stop Step2.1—Oxidation of HMF to FDCA

Oxidation of HMF to FDCA experiment was carried out in a 1 L titaniumreactor (Autoclave Engineers, Serial #85-00534-1). The reactor wascharged with 440 mL of acetic acid, 23 mL of water, 6.2511 g ofcobalt(II) acetate tetrahydrate, 0.44 g of manganese(II) acetatetetrahydrate, and 698 μL of hydrobromic acid (48%). The unit waspressurized to 450 psig using air, and heated to a reaction temperatureof 180° C. Air was then sparged in through a dip tube which had eight229 μm diameter holes at a rate of 4.5 slpm when reactor temperaturereached 170° C. Nitrogen was fed to the inlet of the condenser at 4.5slpm. When desired temperature was achieved, the HMF feed was pumpedinto the reactor at a rate of 3.6 mL/min through another dip tube whichwas positioned by the reactor impeller. The composition of the HMF feedfor this run was 4.48 wt % HMF, 5.18 wt % AcMF, 0.15 wt % HMF dimer and5.4 wt. % humins. This addition was performed over a 45 min period.After the addition was complete, the reactor was cooled to roomtemperature and depressurized. The FDCA solids were then vacuum filteredand dried in a vacuum oven at 75° C. and 200 to 300 torr to give 13.1 gof crude FDCA (with a molar FDCA yield of 80.2%). A detailed analysis ofthis sample is given in Table 3. This sample is referred as sample 2.1.

Step 2.2 Esterification of FDCA to FDME

FDCA sample 2.1 was esterified in a 75 mL mini Parr reactor model 5050equipped with an IKA RCT Basic hotplate stirrer. 8 g FDCA sample 2.1, 32g methanol and TFE stir bar were added to the reactor. The reactor wasplaced in an aluminum block and was kept insulated. The reactor was thenpurged a minimum of 5 times with nitrogen. At room temperature, 300 psiof N₂ was introduced in the reactor head. The reactor was then heated toa temperature of 200° C. and both the temperature and pressure weremonitored. After 4 hr, heat was turned off and the reactor was allowedto cool to room temperature. The pressure was then released and thereactor was opened. The reactor contents (containing mainly FDMEproduct) were removed and transferred to an aluminum pan. Solids weredried overnight on aluminum pan (while methanol was evaporated). Thedried solids were then analyzed using HPLC and SEC. This sample of thecrude methyl ester of FDCA was referred as Sample 2.2 in Table 3. Aportion of the crude methyl ester Sample 2.2 was sublimed using theprocedure given above. The sublimed top and bottom samples are labeled2.3-1 and 2.3-2 in Table 3.

TABLE 3 Results obtained after the analysis of solids collected at theend of stability tests Wt. % Wt. % Wt. % Wt. % Humins Sample L* b* FDMEFDMME FDCA FFME (ppm) 2.1 (after 90.46 25.47 0.00 0.00 98.79 0 41oxidation) 2.2 (after 91.12 25.69 82.85 15.73 0.64 0.51 455 esteri-fication 2.3-1 (after 98.38  2.77 96.85 0.76 0 2.39 <10 subli- mation,collected from top) 2.3-2 (after 87.93 39.77 81.74 16.75 1.51 0.37 52subli- mation, collected from bottom)

From the results shown in Table 3, it is unexpectedly found that impureFDCA solids (containing humins & other impurities) can be converted intoFDME which can be further purified with sublimation. The purified FDMEobtained at the end of sublimation does not contain any humins and alsohas L*>98 and b*<3. The FDCA solids obtained at the end of oxidationreaction contained some FFCA (1.21 wt. %) impurity. This impurity wasconverted to FFME during the esterification step. In the example 2, theFFME impurity was also removed during the sublimation of the FDME andthis resulted in imparting some color to the FDME solids.

What is claimed is:
 1. A process comprising: a) oxidizing a feedstockcomprising hydroxymethyl furfural and humins to produce a mixturecomprising crude humins-containing 2,5-furan dicarboxylic acid; b)separating the mixture to obtain a solid 2,5-furan dicarboxylicacid/humins composition; c) esterifying the solid 2,5-furan dicarboxylicacid/humins composition with an alcohol to produce a crude ester of2,5-furan dicarboxylic acid; and d) purifying the crude ester of2,5-furan dicarboxylic acid obtained in step c) by distillation or bysublimation to produce a purified ester of 2,5-furan dicarboxylic acidsubstantially free of humins.
 2. The process of claim 1 wherein thepurified ester of 2,5-furan dicarboxylic acid is 2,5-furan dicarboxylicacid dimethyl ester.
 3. A process comprising: i) oxidizing a feedstockcomprising hydroxymethyl furfural and humins in a solvent to produce amixture comprising crude humins-containing 2,5-furan dicarboxylic acid;ii) filtering the mixture under high temperature to obtain a filtratecomprising 2,5-furan dicarboxylic acid substantially free of humins,wherein the filtration temperature is sufficiently high to keep the2,5-furan dicarboxylic acid soluble in the solvent; iii) esterifying the2,5-furan dicarboxylic acid substantially free of humins with an alcoholto produce a crude ester of 2,5-furan dicarboxylic acid; and iv)purifying the crude ester of 2,5-furan dicarboxylic acid obtained instep iii) by distillation or sublimation to obtain a purified ester. 4.The process of claim 3 wherein the ester of 2,5-furan dicarboxylic acidis 2,5-furan dicarboxylic acid dimethyl ester.
 5. The process of claim 3wherein the high temperature of the filtration step ii) is in the rangeof from 50° C. to 200° C.
 6. The process of claim 3 wherein the solventis acetic acid or acetic acid and water.
 7. The process of claim 3wherein the purified ester of 2,5-furan dicarboxylic acid comprises lessthan 100 ppm humins as determined by size exclusion chromatography, a b*value of less than 3 as determined by LAB color measurement, and greaterthan 99 percent by weight of the diester of 2,5-furan dicarboxylic acid,based on the total weight of the purified ester of 2,5-furandicarboxylic acid.
 8. The process of claim 3, wherein at least a portionof the alcohol is replaced with an alcohol source.
 9. The process ofclaim 1 wherein the purified ester of 2,5-furan dicarboxylic acidcomprises less than 100 ppm humins as determined by size exclusionchromatography, a b* value of less than 3 as determined by LAB colormeasurement, and greater than 99 percent by weight of the diester of2,5-furan dicarboxylic acid, based on the total weight of the purifiedester of 2,5-furan dicarboxylic acid.
 10. The process of claim 1,wherein at least a portion of the alcohol is replaced with an alcoholsource.